This invention relates to a method and apparatus for the improved removal of molded ophthalmic lenses from the mold in which they are produced. In particular, this invention is suited to molded ophthalmic lenses such as hydrogel contact lenses, although the method is also suitable for other small, high-precision ophthalmic lenses such as intraocular lenses.
As the ophthalmic lens industry has grown, and in particular the industry related to supplying contact lenses that are provided for periodic frequent replacement, the number of lenses that need to be produced has increased dramatically. This has required manufacturers to strive for methods and apparatus that can be adapted to automated practices and perform with consistency.
It is known in the art to make ophthalmic lenses such as soft contact lenses, by forming a monomer or monomer mixture in a mold such as one made from polystyrene or polypropylene.
Examples of this art can be found in U.S. Pat. Nos. 5,039,459, 4,889,664 and 4,565,348. Discussed therein is the requirement for a polystyrene mold that the materials, chemistry and processes be controlled so that the mold portions do not require undue force to separate by sticking to the lens or to each other.
In contrast to the above polystyrene molds, another example is the use of polypropylene or polyethylene molds such as that described in U.S. Pat. No. 4,121,896.
A particular problem is that the mold portions usually are surrounded by a flange, and the monomer or monomer mixture is supplied in excess to the concave mold portion prior to the mating of the molds. After the mold portions are placed together, defining the lens and forming an edge, the excess monomer or monomer mixture is expelled from the mold cavity and rests on or between the flange of one or both mold portions.
Upon polymerization this excess material forms an annular flange or ring around the formed lens.
The prior art process for separating the mold portions and removing the lens consists of preheating, heating, prying and removal. Hot air provides the heating, mechanical leverage the prying, and the removal is manual. Heating the mold by convection is not an efficient heat transfer technique. From the time a mold enters the heating apparatus until the back mold portion is completely removed requires on the order of one minute.
The present method for removing the lens is to apply heat to the back mold portion by means of a heated air stream. The heating of the back mold portion is done in two steps: a preheat stage and a heat/pry stage. In the heat/pry stage the mold is clamped in place and pry fingers are inserted under the back mold portion. A force is applied to each back mold portion during a heating cycle.
When the required temperature has been reached, the back mold portion breaks free and one end is lifted by the pry fingers. After the back mold portion has been detached from the front mold portion on at least one side, the mold then exits the heater. The back mold portion and the annular flashing is then totally removed.
It is also possible to impinge hot or cold air on the outer surface of the front mold portion, to achieve other thermal gradients. The heated air is blown on the exterior of the back mold portion where it transfers heat to the upper surface of the lens. Heat is transported through the back mold, the molded lens, and front mold by thermal diffusion.
While the aforementioned method has some efficacy in assisting the removal of the lens between the mold portions, the temperature gradient achieved from the heated back mold portion, across the lens to the front mold portion is relatively small. The shortcomings in this approach result from the way heat is delivered to the mold portion. The constant temperature air stream heats the exterior surface of the back mold portion more rapidly, while thermal conduction transfers heat to the lens surface. The only way to increase the thermal gradient is to transfer heat faster, but this would cause the back mold portion to become too soft for the lift fingers to engage.
As stated above, this method has not been entirely satisfactory because the thermal gradient induced is not sufficient to fully and repeatedly separate the mold portions.
It is, therefore, an object of the present invention to provide a method and apparatus that can easily and repeatably separate the mold portions without damaging the lens.
It is another object of the invention to perform such separation without excessive environmental heating or waste of energy.
It is a further object of the present invention to provide a method and apparatus whereby a high temperature gradient can be applied across at least one of the mold portions.
These and other objects are attained by the present invention described as follows.